Utilization of coal as an energy source in the U.S. has been hindered by its content of sulfur and ash, which produce undesirable emissions on combustion and increase furnace maintenance, and by the fact that, as a solid, coal is more difficult to handle than liquid or gaseous fuels. For this reason, conversion of coal into liquid or gaseous "synthetic fuels" has, and continues to be the subject of intensive research and development work. In liquefaction, the overwhelming majority of the effort has been on high pressure hydrogenation, sometimes called hydroliquefaction, processes. These processes are relatively expensive, and the cost of the synthetic crude oil they would produce is in the same range ($20--$40/bbl) is the natural petroleum it would replace. These high costs are due to the high pressure and temperatures required, use of expensive stainless steel to resist corrosion and erosion, and the consumption of expensive hydrogen by the process.
At the present time, there is a major difference in price in the United States between low sulfur, nearly ash-free fuels (so-called "compliance" fuels), and surface mined coal, the former selling for about four times the latter, per unit of heating value.
A very attractive alternative to hydroliquefaction lies in the extraction of nearly ash-free carbonaceous solids from coal by alkaline solvents or mixtures under relatively mild conditions. One example of such a process is described by E. P. Stambaugh in U.S. Pat. No. 4,121,910. This process uses aqueous alkalies at elevated temperatures (such as 270.degree. C.) and pressures, and after acid-washing, the ash content of the product is reduced to the range of 0.5-1.0%.
The extraction of coal by organic bases, particularly ethylene diamine and pyridine has also been described extensively in the scientific literature, but has not been used commercially. These particular solvents are relatively expensive and difficult to remove completely from the coal, so that the cost of solvent makeup is large.
Coal extracts obtained with organic bases are, however, lower in ash than those produced by aqueous alkali; ash values of 0.1 WT% or less have been achieved. A high degree of sulfur removal is not achieved, but since U.S. sub-bituminous coals and lignite are naturally low in sulfur, a high value, low sulfur, nearly ash-free product could be produced, which with proper selection and operation of solid-liquid separation equipment, could produce a fuel suitable for use in automotive diesel engines.
An objective of this invention is to describe a commercially viable process for extraction of low rank coals with low-molecular weight primary amines to produce a nearly ash-free extract, with nearly complete solvent recovery and, therefore, reduced cost.